Bone anchors may be used in orthopedic surgery to fix bone during healing or during a fusion process. In spinal surgery, bone anchors may be used with spinal fixation elements, such as spinal rods, to stabilize multiple vertebrae either rigidly, in which no relative motion between the vertebrae is desired, or dynamically, in which limited, controlled motion between the vertebrae is desired. Fixation elements can help to support the spine in a desired alignment, for example by defining a shape towards which a deformed spine is to be corrected. Attaching the vertebrae to the fixation element causes vertebrae which are out of position to be drawn towards the fixation element, so that they can then be retained in a correct alignment against forces imposed by soft tissue tending to revert the configuration of the spine towards the deformed shape. Correction of the spinal deformation can involve application to the vertebrae of translational forces, torsional forces, or combinations thereof to cause vertebrae to translate and/or rotate.
Surgical procedures using bone anchors often require that the bone anchor be secured to the bone at a predetermined angle to a surface of the bone. Traditional bone anchors can include a shaft having a spherical head that is polyaxially seated in a receiver member and that can be secured at a fixed angle to the receiver member by a compression member. To reduce a risk that the compression member is misplaced or dropped into a surgical incision in a patient before the compression member is secured proximally of the head, traditional methods can require deformation of the receiver member against the compression cap. The deformation process, called “swaging,” compresses the receiver member against the compression cap to substantially prevent relative movement therebetween. However, the swaging process can weaken the structure of the bone anchor and can increase the expense and the time required for manufacturing of the bone anchor.
Accordingly, there remains a need for improved methods and devices for bone anchor fixation.